Cryogenic particle blast systems are well known. Systems and the associated component parts are shown in U.S. Pat. Nos. 4,947,592, 5,109,636 and 5,301,509, all of which are incorporated herein by reference. Such systems include a source of cryogenic particles, usually pellets which are typically made of carbon dioxide or any other suitable cryogenic material which preferably sublimes upon impact with the blasting target so that there is no residual particle material to be removed. Such particles are particularly susceptible to degradation due to impacts and direction changes in their flow path. The particles are delivered to a blast nozzle, usually being transported by any suitable transport fluid, typically a gas such as air, carbon dioxide or nitrogen, through a delivery hose. In order to impart as much velocity to the particles to maximize their impact on the blasting target, the exit nozzles are typically converging-diverging supersonic nozzles. Examples of supersonic nozzles are described in U.S. Pat. No. 5,050,805, which is incorporated herein by reference.
The goal of the nozzle is to accelerate the flow, including the entrained particles, to as high a velocity as possible, subject to the design and functional constraints. However, the speed of the transport gas is usually greater than the speed of the entrained particles due to the particles having a mass which is significantly greater than the mass of the gas and being susceptible to decelerating impacts. When the gas and entrained particles are introduced into the entrance of the blast nozzle, the flow transitions from the circular cross-section of the delivery hose to the non-circular cross-section of the blast nozzle. With the conventional prior art converging-diverging blast nozzles, this transition occurs in two dimensions, as the orthagonal dimensions of the nozzle throat were less than the diameter of the delivery hose. In the conventional prior art diverging-converging blast nozzles, the height dimension of the nozzle (i.e., the separation distance of the side support walls) reaches its minimum at the nozzle throat, and remained at its minimum for the length of the nozzle downstream of the throat.
In the conventional prior art converging-diverging blast nozzles, the desire to maximize blast swath width, established by the separation between the diverging nozzle walls, leads to the minimization of the nozzle side wall separation distance. The closeness of the side support walls, maintained downstream of the throat, creates boundary layer viscosity effects which interfere with the proper expansion of the flow, resulting in denigrated flow performance. In such conventional prior art converging-diverging blast nozzles, the constrained particle path movement and associated particle to particle collisions, with the increased side wall viscous turbulence and associated increased particle to nozzle side wall collisions, results in a substantial loss of particle mass. There is also a non-uniform velocity distribution at the exit of the blast nozzle, with the maximum exit velocity occurring at the jet centerline, and lower velocities at either edge of the swath.
In order to attain the desired kinetic energy in conventional prior art diverging-converging blast nozzles, the flow velocity must be increased sufficiently to overcome the deleterious effects of the nozzle design. However, as flow velocity is increased, the particle collisions are increased, and particle mass is decreased further. This results in declining efficiency as flow velocity is increased. It also produces significant increases in noise, which is proportional to velocity to the eighth power. Noise from the blast nozzles is a significant problem, both for the person using the system and for anyone nearby.
In addition to these undesirable functional characteristics of the conventional prior art converging-diverging blast nozzles, there are certain manufacturing constraints which affect not only the cost of producing the nozzles, but the performance of the nozzles. As described below, conventional prior art converging-diverging blast nozzles require symmetry of the opposing diverging walls about the jet centerline, in order for expansion waves to reflect off of the corresponding opposite expansion wave. Attaining and maintaining the necessary symmetry requires that both walls be manufactured to very close tolerances. Deviations from the theoretical symmetrical nozzle results in off design nozzles.
There are other performance related issues associated with conventional prior art converging-diverging blast nozzles. Overexpansion and underexpansion of the nozzles will cause oblique shock waves to occur in the flow. At the operating extremes of overexpanded or underexpanded nozzle flow, a characteristically strong normal type shock wave formation may occur internal or external to the nozzle geometry, respectively. As the delicate entrained particles cross such shock waves, the particles break up.
Additionally, unlike the more customary particle blasting media, such as sand, carbon dioxide particles are significantly more sensitive to self destruction prior to reaching the blasting target. The higher the kinetic energy of the entrained carbon dioxide particles as they pass through the system, the higher the likelihood of particle breakage and mass reduction/sublimation during changes in flow path direction. When a conventional angled nozzle geometry is used for blasting restricted access regions, there is a significant increase in particle break up and reduction in performance attributable to high kinetic energy impacts on the internal nozzle geometry. These performance losses are a direct consequence of not being able to aerodynamically vary the direction of flow exiting from the nozzle from its "in-line" direction.
Thus, there is a need for blast nozzles which can impart significant velocity to particles without excessive loss of particle mass. The exit velocity distribution should be uniform across the entire swath of the exiting flow. The flow direction should be capable of being aerodynamically mined, preserving particle mass. The nozzles need to be quieter and easier and less costly to manufacture than the conventional prior art converging-diverging blast nozzles.